The present invention relates to a technology for pulse shaping, treatment of non-linearity and monitoring in optical communication networks, preferably in optical fiber links. The present invention is a Continuation-In-Part to a U.S. patent application Ser. No. 09/780,572, filed Feb. 12, 2001.
Three basic physical factors, that are known as limiting the achievable bit-rate in optical communication links, are chromatic dispersion, power losses and non-linearity. It is well known that power losses can be compensated by all-optical Erbium-doped or Raman amplifiers periodically installed into a long fiber link. Dispersion can also be compensated by means of periodically inserted relatively short elements with the opposite sign and large absolute value of the dispersion, which makes it possible to have the average dispersion nearly equal to zero. As such dispersion-compensating elements, a specially fabricated fiber, or very short pieces of a fiber with the Bragg grating written on it, may be used.
Nonlinearity, which manifests itself as a nonlinear phase shift accumulated by a light signal while being transmitted via an optical fiber, is generated by the so-called Kerr effect in glass. Owing to this effect, the refraction coefficient of the optical material changes with the intensity of the optical signal according to the following formula:
n=n0+K|E|2,xe2x80x83xe2x80x83(1)
where K is the Kerr coefficient.
WO 00/49458-A1 describes a method and an apparatus for compensating optical non-linearity in optical devices and transmission systems. Two second order interactions are cascaded in phase-mismatched second harmonic generation to accumulate a non-linear phase shift of a fundamental wave. The non-linear phase shift can be set to provide a desired amount of non-linearity compensation. Compensation takes place in a compensating medium having a negative effective non-linear refractive index at the design operating conditions of the compensating medium. Compensators incorporating these principles may be incorporated as passive or active components in optical transmitters, repeaters or receivers. Active components may be tuned by varying the operating condition of the compensating medium, for example by controlling temperature or applied stress. Embodiments of the invention use the compensator as pre- or post-compensators in an optical amplifier, to eliminate or reduce self-phase modulation in the optical amplifier that occurs as a result of the Kerr effect.
C. Pare et al. in their paper xe2x80x9cSplit compensation of dispersion and self-phase modulation in optical communication systemsxe2x80x9d (Optics Letters, 1 Apr. 1996, Vol 21, No. 7, p. 459-461, Opt. Soc. of America) discuss an idea of alternating the sign of the non-linearity along with the sign of the local dispersion by using a (generally, unspecified) medium exhibiting simultaneously a negative Kerr coefficient and specially tailored dispersion. The authors briefly mention that available non-linear media with a negative Kerr coefficient may be semiconductor wave-guides or media utilizing the cascading mechanism. The authors further point out that, though these materials are only available in the form of short samples with the size xcx9c1 cm, the non-linearity of the media might be strong enough to compensate for kilometers of low fiber non-linearity, using pre-amplification if necessary.
It is necessary to note that their estimate was too optimistic: in fact, the semiconductor wave-guides are not acceptable at all, due to the strong two-photon absorption in them; as for the SHG materials, a realistic estimate shows that, in order to compensate the non-linear phase shift accumulated in a typical span of the fiber xcx9c50 km long, the necessary optical path in the second-harmonic-generating material must be no less than xcx9c5 m.
According to one possible way of the full signal restoration discussed in the paper, the dispersion compensation and negative Kerr effects must occur simultaneously, using, for example, a grating structure created on a non-linear wave-guide with a negative Kerr coefficient. Another possible way proposed in the article was to split the compensation process, i.e., the dispersion compensation can be applied first and then, in the next step, the Kerr-induced non-linear effects would be cancelled.
The SHG media known in the art can be represented, inter alia, by nonlinear optical crystals capable of producing higher harmonics of an optical signal from its fundamental harmonic. Such crystals, for example potassium titanyl phosphate (KTP), potassium dihydrogen phosphate (KDP), barium borate optical crystals (BBO) and the like have found their use in various types of laser generators. Examples of such systems can be found in JP 08201862 A2, U.S. Pat. No. 6,047,011, and others.
Notwithstanding the possible degree of the compensation of the dispersion and nonlinearity, they cannot be completely neglected, as they alter the shape of pulses on which the standard non-return-to-zero (NRZ) format of the data transmission in fiber-optic links is based. Ideally, a pulse representing a xe2x80x9conexe2x80x9d bit of data must have a rectangular shape. In reality, the nonlinearity and dispersion convert it into a smoothed signal which is usually close to a Gaussian. The deviation of the data-carrying pulses from the ideal rectangles gives rise to problems produced by overlapping of their extended xe2x80x9ctailsxe2x80x9d belonging to adjacent pulses. The tail overlapping of such tails may give rise to the appearance of parasitic maxima between the xe2x80x9conexe2x80x9d-bits, which poses an additional factor limiting the achievable bit-rate, known as inter-symbol interference (ISI). While a partial solution to this problem may be provided by the above-mentioned dispersion compensation, only strong reshaping of the Gaussian pulses (i.e., periodic restoration of the desired near-rectangular form) would provide for a complete solution of the ISI problem.
T. Zhang and M. Yonemura, in the paper xe2x80x9cPulse Shaping of Ultrashort Laser Pulses with Nonlinear Optical Crystalsxe2x80x9d in Jpn.J.Appl.Phys., Vol. 38 (1999), pp.6351-6358, describe a technique which uses a time-delay optical crystal and a Type-II KDP optical crystal for pulse shaping of a set of two ultrashort pulses carried by the fundamental harmonic. In order to achieve pulse shaping, the interacting pulses must first satisfy the condition that the group velocity of the second-harmonic wave is close to the average group velocity of the two fundamental-harmonic pulses. If this condition is met, pulse shaping is possible by correctly selecting the fundamental intensity, intensity balance, delay time and crystal thickness.
Neither of the above-mentioned references propose a practical method/device for pulse shaping and compensation of non-linearity in fiber-optic links having various lengths, values of the fiber etc.
Further, there is a known technique for monitoring of optical pulse transmission by splitting the pulse signal and obtaining information on the transmission parameters from a minor split out portion of the signal.
It is the objective of the invention to provide a method, a device and a system for pulse shaping, control of non-linearity and/or monitoring in telecommunication fiber links.
According to a first aspect of the invention, the above object can be achieved by providing a method for handling an optical pulse signal, the handling including at least one of operations for: pulse shaping, treatment of nonlinearity and monitoring, the method comprising steps:
providing a signal handling device capable of performing a cascaded second harmonic generation (SHG) with respect to a particular fundamental harmonic (FH),
selecting an optical path length in said signal handling device, suitable for performing at least one of said operations with respect to an incoming optical pulse signal carried by a wavelength defined by said particular fundamental harmonic (FH),
conveying the incoming optical pulse signal carried by said wavelength along the selected optical path in said signal handling device,
obtaining from said signal handling device at least one output optical pulse signal from a list comprising:
an output optical pulse signal at the fundamental harmonic (FH), wherein the treatment of nonlinearity and/or the pulse shaping are performed,
an output optical pulse signal at the second harmonic (SH) for further monitoring it and judging about said input optical pulse signal.
In one preferred version of the method enabling performing the operation of nonlinearity treatment, the method comprises selecting such an optical path length for conveying the incoming optical pulse signal with a known amplitude via the signal handling device, that is substantially close to the length upon passing which the output optical pulse signal at the fundamental harmonic (FH) reaches the maximum peak power.
In another preferred version of the method, ensuring performing the operation of pulse shaping, the method comprises selecting such an optical path length for conveying the incoming optical pulse signal with a known amplitude via the signal handling device, that is substantially close to the shortest optical path length upon passing which the output optical pulse signal at the fundamental harmonic (FH) reaches the maximum peak power.
In yet a further version of the method, allowing for the monitoring operation, the method comprises selecting such an optical path length for conveying the incoming optical pulse signal via the signal handling device, enabling obtaining from said device the output optical pulse signal at the second harmonic (SH) with a non-zero peak power for monitoring the incoming optical pulse signal carried by the fundamental harmonic (FH).
Principles of selecting the optical path length will be explained in the detailed description of the invention.
To obtain a required optical path length, the method preferably comprises passing the signal along a multi-segment trajectory in said device, thereby arranging an extended optical path.
One possibility to attain the selected optical path length is to convey the incoming optical pulse signal via a multi-segment xe2x80x9czig-zagxe2x80x9d trajectory by arranging one or more internal reflections in the signal handling device.
In the method, the signal handling device is based upon an element selected from the following non-exhaustive list including: a second harmonic generating (SHG) optical crystal and a second harmonic generating (SHG) polymer fiber, both known as elements producing nonlinearity or non-linear phase shift.
According to the most preferred version of the method, it further comprises a step of ensuring that the sign of the Kerr effect created by said element to said wavelength defined by the fundamental harmonic is negative. In this case, the method enables the nonlinearity treatment in the form of compensation of the positive nonlinearity usually accumulated in said incoming optical pulse signal due to conventional positive Kerr effect of optical fibers.
It should be emphasized that, unlike the nonlinearity compensation, the pulse shaping and the monitoring can be achieved by using the device producing nonlinearity of any sign. Likewise, a positive nonlinearity adjustment being a specific case of the nonlinearity treatment is provided, when necessary, using the device inducing the positive Kerr effect.
The method is most efficient for gradual compensation of the nonlinearity and/or gradual pulse shaping in the fiber optic link with optional simultaneous signal monitoring, and comprises an additional step of conveying the outgoing optical signal via a chain including at least one additional signal handling device, and wherein the devices in the chain are spanned by sections of the optical fiber link. In other words, if more than one said devices are inserted in the link and spaced from one another, each of them will contribute to the optical signal handling from the point of nonlinearity treatment, pulse shaping and/or signal monitoring.
By selecting the kind of the device(s), the total length of the optical path in said one or more device(s), and lengths of said one or more sections of the optical fiber link, the obtained results of the signal handling can be adjusted.
The proposed method is also applicable to a case of multi-channel transmission of optical data, where each of the optical channels transmits a specific optical signal at a particular optical wavelength. Usually, the SHG devices are capable of generating second harmonics to a limited spectral range of respective fundamental harmonics defined by wavelengths close to one another. Therefore, the method may be applied to the WDM (Wavelength Division Multiplexing) transmission format, where wavelengths of the optical channels slightly differ from each other.
The proposed method can be utilized in a multi-channel transmission system by performing operations of the basic method with respect to each particular optical channel.
According to one version, the optical pulse signals of different said optical channels are applied to and conveyed via respective different said signal handling devices.
In an alternative version of the method, it comprises conveying the optical pulse signals of different channels via one and the same common signal handling device.
In a further, more promising version, the optical pulse signals of different said optical channels are applied and conveyed via respective different layers of one and the same common pulse treatment device.
The last two versions are suitable for such transmission formats where the wavelengths of different optical channels are close to one another, and provided that the common signal handling device performs its SHG cascaded function in response to the wavelength of each of said multiple optical channels.
If results of the pulse treatment are nonuniform for different optical channels in the multi-channel transmission (which is usually the case), optical channels with better results (say, better compensation of nonlinearity/more effective pulse shaping) can be used for transmitting information having higher priority.
In accordance with a second aspect of the invention, there is provided a device for handling an optical pulse signal from the point of at least one of the following operations: pulse shaping, treatment of nonlinearity and signal monitoring,
the device being capable of performing a cascaded second harmonic generation (SHG) with respect to a particular fundamental harmonic (FH),
the device being characterized by such an optical path length selected for an incoming optical pulse signal carried by a wavelength defined by said particular fundamental harmonic (FH), that upon conveying said incoming optical pulse signal along the selected optical path, the device enables obtaining at least one output optical pulse signal from a list comprising:
an output optical pulse signal at the fundamental harmonic (FH), wherein the treatment of nonlinearity and/or the pulse shaping are performed,
an output optical pulse signal at the second harmonic (SH) suitable for further monitoring and judging about said input optical pulse signal.
The signal handling device comprises a second-harmonic-generating (SHG) element, preferably constituting an SHG optical crystal selected from a non-exhaustive list comprising KTP, KDP and BBO.
It should be noted that the Inventors are first to propose design of a device for handling an optical pulse signal, if applied at a particular wavelength, from the point of at least one of the following operations: pulse shaping, treatment of nonlinearity and signal monitoring, wherein the device comprising
an SHG element for performing a cascaded Second Harmonic Generation with respect to a Fundamental Harmonic (FH) defined by said particular wavelength,
said element being covered by mirror surfaces at least at its two opposite facets and leaving at least two windows at said opposite facets for an incoming optical beam and an outgoing optical beam respectively, the arrangement being such to arrange one or more internal reflections of the optical beam if passing between said two windows, thereby providing an extended optical path.
The extended optical path preferably has a length enabling obtaining an outgoing optical pulse signal on the fundamental harmonic (FH) with a peak power close to maximum and/or an outgoing optical pulse signal on the second harmonic (SH) with a non-zero peak power.
According to one specific implementation, the element (preferably the SHG crystal) has a cubic form and is covered at its two opposite facets by mirror surfaces (for internal reflection), leaving two windows at said opposite facets for an incoming optical beam and an outgoing optical beam respectively, the windows being arranged to obtain an extended optical path of the optical beam through the crystal.
In the preferred embodiment of the device, it is adapted for altering the total length of the multi-segment trajectory, thereby enabling adjustment of the nonlinearity compensation, of the pulse shaping, and/or possibility of the signal monitoring. To this end, the device may have more than two optical ports for incoming and outgoing beams, thus enabling selection and activation of any pair of such ports for a specific length of the trajectory. Alternatively or in addition, the device may be provided with collimators associated with the optical ports and serving for adjusting the incident angle of the light beam.
The device may be utilized for signal handling in a multi-channel transmission format, wherein each of the channels transmits an optical signal at a particular wavelength, said device being capable of Second Harmonic Generation with respect to the wavelengths of more than one channels of said format.
According to one particular embodiment, the pulse treatment device having the SHG property with respect to wavelengths of a number of the multiple optical channels is divided into a number of layers for respectively conveying there-through optical signals of the different optical channels. Ideally, the device serves all the multiple channels.
This embodiment is suitable for the WDM transmission format where the wavelengths of different optical channels are close to one another, (and provided that the common pulse treatment device performs its SHG property in response to at least a number of wavelengths of the respective multiple optical channels).
The layers may be separated either geometrically, or physically, say by optical gratings serving to prevent wavelengths of adjacent optical channels from passing via a particular layer. Actually, such physical separating means provide wavelength filtering.
The device is preferably integrated with an optical amplifier and is preferably placed immediately after said amplifier. The amplifier is usually utilized for adjusting the amplitude of the pulse applied to the device. In practice, the proposed device may form part of an optical network node.
According to an additional aspect of the invention, there is also provided a method for designing a signal handling device, which will be described, with the aid of drawings, in the detailed description of the invention.
Finally, there is proposed a suitable system for handling signals passing via optical fiber links from the point of pulse shaping, nonlinearity treatment and/or monitoring, the system comprising two or more signal handling devices as defined above, inserted in one or more optical fiber links and operative to perform pulse shaping, nonlinearity treatment and/or monitoring with respect to at least an optical pulse signal transmitted via one optical channel.
Adjustment of the systems"" operation can be achieved by
a) reconfiguring the signal handling devices (selecting input-output ports, regulation of the collimators, etc.);
b) introducing additional devices or removing excessive devices;
c) changing distances between the devices and other elements of the link(s).
Further aspects and details of the invention will become apparent from the following description.